JPS63283195A - Radio wave absorber - Google Patents

Abstract

PURPOSE:To improve the practical utility by laminating a specific resistance film on the upper surface of a dielectric layer of predetermined thickness. CONSTITUTION:A dielectric layer 2 having lambdaepsilon/4 of thickness, where lambdaepsilon is radio wave length in a dielectric layer, is provided on a conductive plate 1. A resistance film 3 in which semiconductive ceramic carbon fiber having 0.1-100 mum of diameter, 10-100 of aspect ratio and 10<-3>-10<2>OMEGA.cm of volume resistivity is dispersed to be mixed in polymer substance, and a protective film 4 are laminated on the layer 2 to form a radio wave absorber. This absorber is preferably matched in space impedance to provide a light weight, a thin type, high durability, weather resistance and waterproofness against the radio wave of wide frequency band, and good workability by a coating method, thereby improving its practical utility.

Description

DETAILED DESCRIPTION OF THE INVENTION Industrial Procedure 1 The present invention relates to a λ/4 resonance type radio wave absorber using a resistive film made of semiconductive ceramic short fibers.

We live in a highly information-oriented society, where we are flooded with various information media, and the number of individual media continues to increase.

Communication, broadcasting, observation and detection using radio waves are no exception, and include conventional AM broadcasting, shortwave broadcasting, FM broadcasting, V
In addition to radio broadcasting and communications such as HF television broadcasting, UHF television broadcasting and microwave multiplex radio, maritime radio navigation, aviation radio navigation, land and maritime mobile radio, satellite broadcasting and data communication using communication satellites and meteorological satellites. As a result, radio waves in all frequency bands are now being used extensively, leading to problems such as radio wave interference, frequent radio interference, and malfunctions. It has become a major social and environmental problem, and the so-called EM
C (Electro Magnetic Co+npa
There is a strong need for countermeasures against this issue.

An example of this EMC countermeasure is the installation of radio wave absorbers. Various types of radio wave absorbers have been proposed and implemented, but the most effective radio wave absorber as an EMC countermeasure is one that has a resistive film on the side in which the radio waves arrive, and a conductive plate on the back side. One example is a so-called λ/4 resonant type radio wave absorber having a laminated structure in which a dielectric material with a thickness of approximately λε (here, λE indicates the wavelength of the radio wave within the dielectric material) is laminated on the dielectric material. can.

The radio wave absorption mechanism of this λ-resonance type radio wave absorber reflects radio waves with exactly half the power of the incoming radio waves onto the resistive film.
This is to cancel out the total power of the opposite-phase radio waves that are transmitted through the dielectric having a thickness of λε and multiple reflected by the conductive plate on the back surface. In order to achieve this, it is desirable that the impedance of the resistive film has a certain value that is slightly smaller than the spatial impedance, and that the resistive film is as thin as possible in terms of shape. It is advantageous in that it can be used widely. Furthermore, the radio wave absorber
It is necessary to have weather resistance and durability to maintain the above electromagnetic properties for a long period of time, to be lightweight and flexible, and to have mechanical properties that are easy to construct.

However, although various materials and manufacturing methods have been proposed for each component of this λ/4 resonance type radio wave absorber, they have advantages and disadvantages in terms of performance, durability, workability, and price, and are still not widely used. The reality is that there is nothing that can be put to practical use.

In particular, the material of the resistive film and its manufacturing method are a problem. Conventionally, the following materials and manufacturing methods have been proposed or implemented, but each of them has the following drawbacks.

a. A film or coating in which conductive carbon or metal filler is dispersed.

b. Resistance coating by carbon baking or nichrome vacuum deposition.

A resistance film made of CO copper sulfide treated acrylic fibers arranged sparsely in a grid pattern.

d. Conductive fiber treated with copper sulfide or metal plating.

e, carbon fiber.

Among these, the film or coating film a has large variations in conductance or transmission loss within the film, making it difficult to match the impedance required for the design of a λ74 resonant radio wave absorber, and also has poor weather resistance. It also lacks durability.

Furthermore, the resistive coating produced by the manufacturing method b is difficult to form uniformly and economically over a large area, and also lacks thermal durability, so that it has not been put to practical use.

Furthermore, in the case of a C resistor film, the lattice spacing must be adjusted one by one in order to match the impedance.
Furthermore, leakage of radio waves from between the grids becomes a problem for short wavelength radio waves.

Furthermore, the conductivity of the conductive fibers d tends to change under high temperature or high humidity conditions, and therefore some kind of protective coating must be applied.

Carbon fiber (e) is difficult to use because its initial conductivity is unspecified, and is expensive, so it is limited to special uses.

Therefore, a resistive film for a λ14 resonance type radio wave absorber that is excellent in performance and economy is desired, and there is a demand for the development of a highly practical radio wave absorber.

The present invention has been developed in view of the above circumstances, and has the ability to easily match the spatial impedance (approximately 377Ω) to incoming radio waves, has excellent electromagnetic properties of durability and weather resistance, and is lightweight and flexible. However, it is an object of the present invention to provide a λ14 resonance type radio wave absorber that can be easily constructed and has high practicality.

In order to achieve the above object, the inventors of the present invention conducted extensive studies and found that the thickness of the layers laminated on the conductive plate was approximately λε/4.
(Here, λE indicates the wavelength of radio waves in the dielectric material) is obtained by dispersing semiconductive ceramic short fibers such as semiconductive alkali titanate short fibers in a polymer material on the top surface of the dielectric layer. The radio wave absorber, which is laminated by laminating resistive films, is lightweight, flexible, and easy to construct, and it absorbs radio waves in a wide frequency range well, and has excellent durability and weather resistance.
It was discovered that such a radio wave absorber can be used for various purposes with sufficient practicality, and the present invention was completed.

Therefore, in the present invention, the thickness of the layer stacked on the conductive plate is approximately λ
A resistive film obtained by dispersing semiconductive ceramic short fibers in a polymeric substance is laminated on the top surface of a dielectric layer of ε/4 (here, λξ indicates the wavelength of radio waves within the dielectric). The present invention provides a radio wave absorber characterized by the following characteristics.

The present invention will be explained in more detail below.

The radio wave absorber of the present invention, as shown in FIG. In some cases, the resistive film is made of a film obtained by dispersing semiconductive ceramic short fibers in a polymeric substance.

Here, the semiconductive ceramic short fibers used for forming the resistance film of the radio wave absorber of the present invention are not necessarily limited, but have a fiber diameter of 0.1 to 100/j1.
1. It is preferable that the shape has an aspect ratio of 10 to 1000 and a specific volume resistivity of 10'''3 to 102Ω, more preferably a fiber diameter of 0.1 to 10Iia,
Aspect ratio 10~300, volume resistivity 10-1~
50Ω/country.

When the fiber diameter is larger than 1004 or the aspect ratio is 1
If semiconductive ceramic short fibers exceeding 0.000 are used, the surface smoothness will decrease, pinholes will easily occur, the mechanical properties will decrease, and it may not be possible to obtain a film that is flexible and has excellent workability. be. In addition, the fiber diameter is 0. When semiconductive ceramic short fibers with an aspect ratio of less than IJs or less than 10 are used, there is no major difference from the case when conventional conductive powder fillers are used, and the highly flat surface is a major feature of the radio wave absorber of the present invention. In some cases, it may not be possible to obtain a very thin resistive film that is oriented in the same direction. Furthermore, the specific volume resistivity is 10-3Ω・1
If the impedance of the resistive film is less than or exceeds ↓02Ω, the impedance of the resistive film should be adjusted to the design value (377Ω) while maintaining good mechanical properties or using a manufacturing method with good reproducibility.
There may be cases where it is difficult to set the value to a value slightly lower than Ω.

As this semiconductive ceramic short fiber, a semiconductive ceramic fiber that satisfies the above-mentioned range in terms of shape and conductivity may be used as is, or a non-conductive ceramic fiber whose shape is within the above-mentioned range may be used. It is also possible to use short ceramic fibers that have been made conductive by a surface conductive treatment method to have semiconductivity within the above range.

A specific example of the semiconductive ceramic staple fiber suitably used as the resistance coating of the radio wave absorber of the present invention is given by the general formula % (where M is an alkali metal, a, b, and X are real numbers, and 0 ≦
a≦8.0≦b≦4.0<x<2) Semiconductive alkali titanate short fibers called reduced alkali titanate or bronze alkali titanate, or the general formula % formula % (wherein M is Alkali metal, a, b and y are real numbers, 0≦
a≦8, 0≦b≦4. Examples include semiconductive alkali titanate short fibers obtained by coating alkali titanate short fibers represented by O<y≦2) with a conductive metal compound such as a metal, a metal oxide, or a metal halide compound.

Regarding the manufacturing method of these semiconductive alkali titanate short fibers, please refer to JP-A-57-103204 and JP-A-58-13512.
9, 58-135130, 60-9005, 61
Various methods are known, such as those described in JP-A-55217 and JP-A-61-167017.

The resistive film of the radio wave absorber of the present invention is obtained by dispersing and blending the above-mentioned semiconductive ceramic staple fibers in a polymeric substance as a binder, and is used as a binder to disperse and blend the semiconductive ceramic staple fibers. There are no particular restrictions on the type of polymer material used, and various materials can be selected depending on the purpose from the viewpoints of ease of manufacture, price, required mechanical strength and durability, weather resistance, etc. may be used, but urethane resin or vinyl chloride resin is generally suitable.

In this case, the blending amount of the semiconductive ceramic short fibers is preferably in the range of 20 to 200 parts by weight per 100 parts by weight of the polymeric substance, and in particular, a blend of 50 to 150 parts by weight achieves the object of the present invention. more preferable to If the blending amount of the semiconductive ceramic short fibers is less than 20 parts by weight, the impedance of the resistive film will be 10 3 Ω or more, which may not match the spatial impedance (approximately 377 Ω). In addition, as mentioned above, λ is resonant type'1
It is desirable that the resistance film of the radio wave absorber be as thin as possible within the range that provides the required impedance, and it is more advantageous to increase the amount of semiconductive ceramic staple fibers, but if the amount exceeds 200 parts by weight In this case, the mechanical strength of the resistive film may decrease, making it practically inferior.

Further, the thickness of the resistive film is preferably 15 to 150 Am, more preferably 30 to 100 Am. As mentioned above, the thinner the resistive film is, the better it is functionally, but if it is less than 15Ia, the mechanical strength will be insufficient and it may be difficult to manufacture a radio wave absorber, and if it is more than 1507a In this case, the ratio of the reactance component to the resistance component among the impedance components of the resistive film becomes too large, and matching with the spatial impedance may not be achieved.

The uniformity of the thickness of this resistive film is one of the important elements in the design of the λ14 resonance type radio wave absorber.The film must be created within the tolerance using the required method in advance, and then applied to the dielectric layer. It is desirable to laminate the material by an appropriate means such as bonding it on top of the material.

When this resistive film is produced industrially, it is usually made continuously as a long product, so that short fiber orientation phenomenon is observed in the physical properties, and the electromagnetic properties may differ in the length direction and the width direction. Therefore, if the plane of polarization of the incoming radio wave is known in advance, such as when the radio wave to be absorbed is horizontally polarized like in television broadcasting or vertically polarized like in personal wireless, the direction of orientation of the resistive film can be adjusted. The design and construction should be done according to the plane of polarization of the radio waves.Also, if the plane of polarization of the incoming radio waves is unknown, two films for resistive coating should be designed and constructed so that the orientation directions of the short fibers are almost perpendicular to each other. pasted on 2
By using N film as a resistive film and setting the impedance of the two-layer film whose orientation has been canceled to a predetermined value, it is possible to create a radio wave absorber that exhibits equal absorption performance for radio waves in all directions, and then install this. . Orientation can also be improved by drilling holes in a single-layer resistive film, the long axis of which is less than half the wavelength of the incoming radio wave, so that they are dense in the orientation direction of the short fibers and coarser in the non-orientation direction. A neutral radio wave absorber can be obtained.

In this case, it is important to make the long axis of the hole less than half the wavelength of the incoming radio wave; if the diameter of the hole is larger than half the wavelength of the incoming radio wave, most of the radio wave will pass through, rendering the resistive film useless.

In the present invention, the above-mentioned resistive film is laminated on the dielectric layer, and the dielectric layer is designed to control the electrical length between the resistive film and the conductive reflector on the back (the path of the incoming radio wave) within the dielectric layer. This dielectric layer is an essential component in order to maintain approximately 1/4 of the wavelength, and the thickness of this dielectric layer is λε14 (here, the thickness of the dielectric layer is λε14 (here, the stone is the thickness of the radio wave within the dielectric). However, the actual radio wave absorber structure has some glue actance component due to the thickness of the resistive film, and may deviate slightly from the theoretical thickness λε, and the range of this deviation is usually λε /4 is about 0.7 to 1.3.

As the material for this dielectric layer, organic foams such as polyethylene, polypropylene, polystyrene, and polyvinyl chloride are preferably used, but polyethylene foams with an expansion rate of 10 to 40 times are particularly preferred from the viewpoint of durability and economical efficiency. is desirable.

The dielectric layer is laminated on the conductive plate, but in this case, the dielectric layer may be laminated on the conductive plate during use, or the dielectric layer may be laminated on the conductive plate in advance. .

Note that various materials such as copper, aluminum, silver, or alloys thereof can be used as the conductive plate on which the dielectric layer is laminated.

When the radio wave absorber of the present invention is actually used,
As shown in Figure 1, in order to further improve waterproofness, durability, weather resistance, or aesthetics, insulating organic An S-retaining layer (reference numeral 4 in FIG. 1) such as a foam sheet, an organic polymer film, or an organic coating film can be formed by bonding or painting.

As explained above, the radio wave absorber of the present invention uses a film in which semiconductive ceramic short fibers are dispersed in a polymeric material as a resistive film, so that it can be used in a range from VHF to UH.
Lightweight and highly durable for radio waves in wide frequency bands of F, SHF, and EHF bands.

It is possible to provide a radio wave absorber that is easy to construct and has excellent radio wave absorption performance, and can be used to prevent ghosts in communication and broadcasting, microwave transmission and reception equipment, and radar equipment for ships and fishing boats.

It can be suitably used in a wide range of areas where it is generally required as a part of EMC countermeasures, such as countermeasures against radio wave interference in the vicinity of satellite broadcasting receivers, countermeasures against false images in bridges and high-rise buildings, etc.

EXAMPLES Hereinafter, the present invention will be specifically explained by showing examples and comparative examples, but the present invention is not limited to the following examples.

[Example 1] 8mm thick aluminum plate laminated on 200mm thick aluminum plate
On the top surface of the nm polyethylene foam (foaming rate 30 times),
Fiber diameter is 0.2~Q, 5 pg, aspect ratio is 2
0 to 1002 100 parts by weight of semiconductive potassium titanate short fibers having a specific volume resistivity of 3Ω·1 are added to 1 part by weight of urethane resin.
A radio wave absorber was produced by dispersing and blending the resistive film into a film having a thickness of 50 p.

[Example 2] 8mm thick aluminum plate laminated on 200mm thick aluminum plate
The upper surface of the polyethylene foam (expansion rate: 30 times) in the picture shows fibers with a diameter of 0.2 to Q,5,cm and an aspect ratio of 2.
0 to 1009 70 parts by weight of semiconductive potassium titanate short fibers with a specific volume resistivity of 1 Ω/cm were mixed with 10 parts by weight of urethane resin
A radio wave absorber was prepared by dispersing and blending the resistive film into a film having a thickness of 40 parts.

[Comparative Example 1] On the top surface of an 8 m thick polyethylene foam (foaming rate: 30 times) laminated on an aluminum plate with a thickness of 200/7111, fiber diameters of 0.2 to Q, 5 pre, aspect ratio 150 parts by weight of semiconductive potassium titanate short fibers having a specific volume resistivity of 20 to 1002 and 104 Ω/cm are dispersed and blended into 100 parts by weight of urethane resin to form a material with a thickness of 100 Ω.
A radio wave absorber was produced by bonding a resistive film formed on the film of -.

[Comparative Example 2] 8mm thick aluminum plate laminated on 200mm thick aluminum plate
80 parts by weight of acetylene black was dispersed and blended into 100 parts by weight of urethane resin on the top surface of the polyethylene foam (expansion rate: 30 times) of I, resulting in a thickness of 200p! A radio wave absorber was produced by bonding the molded resistive film to the M film.

[Comparative Example 3] 8mm thick aluminum plate laminated on 200mm thick aluminum plate
On the top surface of mm polyethylene foam (foaming rate 30 times),
Long diameter 25. 25 parts by weight of Ni flakes and 1 part of urethane resin
A radio wave absorber was prepared by dispersing and blending the resistive film into a film having a thickness of 50,41,111 mm and bonding the resistive film to 0.00 parts by weight.

[Comparative Example 3] 8mm thick aluminum plate laminated on 200mm thick aluminum plate
A woven fabric made of copper sulfide-treated acrylic fibers with a resistivity of 5 x 10-''Ω woven in a lattice pattern with a spacing of 8 m is attached as a resistive film to the top surface of polyethylene foam (foaming rate: 30 times). The body was created.

Next, regarding the radio wave absorbers produced in Examples 1 and 2 and Comparative Examples 1 to 4 above, the radio wave absorption performance of the samples was measured before and after a moist heat durability test of 80°C x 90% RH x 100 hours using the RL arch method. The measurement was performed using a tester in the frequency band of 2.5 to 11 GHz. The absorption peak values are shown in Table 1.

Table 1 Next, FIG. 2 shows a measurement chart of the radio wave absorption performance test conducted on the sample of the absorber of Example 1 before the thermal durability test. From the results shown in FIG. 2, the radio wave absorber of Example 1 has an absorption peak value of approximately 5.2 GHz, and a peak value of 4.5 GHz.
~20dB or more at 6GHz, 10d at 3.5-7GHz
It can be seen that this is a radio wave absorber that exhibits excellent radio wave absorption ability in a wide frequency band with absorption characteristics of B or higher.

[Brief explanation of drawings]

FIG. 1 is a partially omitted sectional view showing an example of the structure of the radio wave absorber of the present invention, and FIG. 2 is a graph showing a measurement chart of a radio wave absorption performance test conducted on the radio wave absorber of Example 1. DESCRIPTION OF SYMBOLS 1... Conductive plate, 2... Dielectric layer, 3... Resistance film, 4... Protective film.

Claims (1)

[Claims] 1. The thickness of the layers laminated on the conductive plate is approximately λ_ε_/_4 (
Here, λ_ε indicates the wavelength of radio waves in the dielectric material) A resistive film obtained by dispersing semiconductive ceramic short fibers in a polymeric material is laminated on the top surface of the dielectric layer. Radio wave absorber. 2. The semiconductive ceramic short fibers have a fiber diameter of 0.1 to 10
The radio wave absorber according to claim 1, which is a semiconductive ceramic short fiber having an aspect ratio of 0 μm, an aspect ratio of 10 to 1000, and a specific volume resistivity of 10^-^3 to 10^2 Ω·cm. 3. The radio wave absorber according to claim 1 or 2, wherein the semiconductive ceramic short fibers are semiconductive alkali titanate short fibers. 4. Claims 1 to 3, wherein the resistive film is a film having a thickness of 15 to 150 μm and containing 20 to 200 parts by weight of semiconductive ceramic short fibers dispersed in 100 parts by weight of a polymeric substance. The radio wave absorber according to any one of paragraphs. 5. The radio wave according to any one of claims 1 to 4, wherein the resistive film is a multilayer film in which the orientation directions of semiconductive ceramic short fibers are laminated so that they are substantially perpendicular to each other. Absorber. 6. The resistive film is a single-layer film in which holes whose major axis is less than half the wavelength of the incoming radio wave are perforated densely in the orientation direction of the semiconductive ceramic short fibers and coarsely in the non-orientation direction. A radio wave absorber according to any one of claims 1 to 4. 7. The method according to any one of claims 1 to 6, wherein a protective layer selected from an insulating organic foam sheet, an organic polymer film, and an organic coating film is laminated on the surface of the resistive film. Radio wave absorber. 8. The dielectric layer according to any one of claims 1 to 7, wherein the dielectric layer is formed of an organic foam selected from polyethylene, polypropylene, polystyrene, polyurethane, and polyvinyl chloride. Radio wave absorber.